Optical transmission fiber with thermal management for high-power applications

ABSTRACT

An optical transmission fiber including a core having a first index of refraction, a cladding material located around the core and having a second index of refraction less than the first index of refraction, a first coating material located around a first portion of the cladding material and having a third index of refraction greater than the second index of refraction, and a second coating material located around a second portion of the cladding material and having a fourth index of refraction less than the second index of refraction.

TECHNICAL FIELD OF THE INVENTION

The present invention is directed, in general, to transmissionfiberoptics and, more specifically, to an optical transmission fiberincluding thermal management for high-power applications.

BACKGROUND OF THE INVENTION

In general, an optical transmission fiber consists of a core, claddingaround the core, and an exterior coating. Conventionally, the claddinghas a refractive index that is less than a refractive index of the core,thereby confining an optical signal within the core. Similarly, thecladding often includes an exterior coating that has a higher refractiveindex than the cladding. Thus, the exterior coating allows any lightenergy that escapes the core to quickly exit the fiber rather thanreentering the core or propagating as a cladding mode and interferingwith the transmission signal propagating therein.

However, while such optical fibers have long been used, this design isnot flawless. For example, in unidirectional applications, mostjunctions between two or more fibers generate coupling loss. In suchconditions, the light energy not properly coupled into the downstreamfiber core can be injected into the cladding of either the upstream ordownstream fiber. For instance, a portion of the input light energy canbe incident on the core/cladding interface at an angle less than thecritical angle of incidence, as provided by Snell's Law. Upon such anoccurrence, this light energy passes from the core and continues throughthe interface between the cladding and the coating, because theconventional coating has a higher index of refraction than that of thecladding. This light energy may be absorbed by the coating or anysurrounding materials (e.g., cabling and packaging materials) andconverted into heat energy. The heat energy can cause localized damageto the optical fiber and surrounding materials, which significantlyreduces the operational life of the fiber. This is particularlyconsequential in high-power applications, such as but not limited tothose where the transmission signal has a power above 0.5 W. In manycases, the surrounding material may have very poor thermal conductioncharacteristics, which compounds the damage caused by the light energyescaping the core, ultimately causing the optical fiber to failprematurely. Of course, any optical components proximate the junctionare also susceptible to the elevated temperatures.

Other applications that encourage such premature optical fiber failureinclude the use of bulk optics. More specifically, bulk optics packagestypically require decoupling a signal from a package input fiber andprocessing the signal with the bulk optics components within thepackage. Thereafter, the processed signal is typically re-coupled to anoutput fiber. However, in such arrangements, coupling losses occurbetween the bulk optics components and/or the ends of the input/outputfibers, such as those attributable to optical misalignment. Again, it islikely that this stray light energy will find its way into the claddingof the input or output fibers (pigtails) and cause the package and/or alocalized portion of the optical fiber to overheat and fail prematurely.

Similarly, work-site obstructions, disadvantageous panel configurationsand other installation/assembly obstacles may result in an opticaltransmission fiber to be permanently configured with a severe bend, suchas one having a radius or kink smaller than about 10 mm. In suchinstances, the severe curvature of the fiber may cause signal energypropagating along the core to be injected into the cladding. Again, theescaping light energy is converted to heat upon leaving the cladding,which can overheat a localized portion of the optical transmissionfiber, resulting in premature failure.

Accordingly, what is needed in the art is an optical transmission fiberthat overcomes the above-discussed problems experienced by conventionaloptical transmission fibers.

SUMMARY OF THE INVENTION

To address the above-discussed deficiencies of the prior art, thepresent invention provides an optical transmission fiber including acore, a cladding material located around the core, and first and secondcoating materials around the cladding material. The first coatingmaterial is located around a first portion of the cladding material andhas an index of refraction greater than an index of refraction of thecladding material. The second coating material is located around asecond portion of the cladding material and has an index of refractionless than the index of refraction of the cladding material.

The foregoing has outlined an embodiment of the present invention sothat those skilled in the art may better understand the detaileddescription of the invention that follows. Additional features of theinvention will be described hereinafter that form the subject of theclaims of the invention. Those skilled in the art should appreciate thatthey can readily use the disclosed conception and specific embodiment asa basis for designing or modifying other structures for carrying out thesame purposes of the present invention. Those skilled in the art shouldalso realize that such equivalent constructions do not depart from thespirit and scope of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention is best understood from the following detaileddescription when read with the accompanying FIGUREs. It is emphasizedthat in accordance with the standard practice in the optics industry,various features may not be drawn to scale. In fact, the dimensions ofthe various features may be arbitrarily increased or reduced for clarityof discussion. Reference is now made to the following descriptions takenin conjunction with the accompanying drawings, in which:

FIG. 1 illustrates a plan view of an embodiment of an opticaltransmission fiber constructed according to the principles of thepresent invention;

FIG. 2 illustrates a plan view of another embodiment of an opticaltransmission fiber constructed according to the principles of thepresent invention;

FIG. 3 illustrates a plan view of yet another embodiment of an opticaltransmission fiber constructed according to the principles of thepresent invention;

FIG. 4 illustrates a plan view of yet another embodiment of an opticaltransmission fiber constructed according to the principles of thepresent invention; and

FIG. 5 illustrates a plan view of an embodiment of an opticaltransmission system constructed according to the principles of thepresent invention.

DETAILED DESCRIPTION

Referring initially to FIG. 1, illustrated is a plan view of anembodiment of an optical transmission fiber 100 constructed according tothe principles of the present invention. The optical transmission fiber100 includes a core 110 and a cladding material 120 located around thecore 110. The core 110 and cladding material 120 may be of conventionalmaterials and construction. As such, the cladding material 120 has anindex of refraction that is less than an index of refraction of the core110, such that a transmission signal (light energy) propagating alongthe core 110 is substantially confined within the core 110. As shown inFIG. 1, the transmission signal propagates within the core 110 in thegeneral direction of the arrow 115.

The optical transmission fiber 100 also includes a first coatingmaterial 130 located around a first portion 120 a of the claddingmaterial 120. The first coating material 130 may also be of conventionalmaterials and construction. The first coating material 130 has an indexof refraction that is greater than the index of refraction of thecladding material 120, such that any light energy that escapes the core110 into the cladding material 120 may quickly exit the claddingmaterial 120 through the first coating material 130. As known to thoseskilled in the art, light energy may escape the optical transmissionfiber 100 as light energy, or it may be at least partially converted toheat energy within the coating material 130 and radiate to the ambientenvironment. In many applications, the energy escaping the opticaltransmission fiber 100 in this manner can be deleterious to materialssurrounding the optical transmission fiber 100, and subsequently to theoptical transmission fiber 100 itself, as discussed above and furtherdescribed below. Those skilled in the art will also recognize thatadditional coatings (not shown) may be formed on the first coatingmaterial 130.

The optical transmission fiber 100 also includes a second coatingmaterial 140 located around a second portion 120 b of the claddingmaterial 120. The second coating material 140 is substantially notlocated over the first coating material 130 but, as those skilled in theart will understand, the second coating material 140 may have additionalcoatings (not shown) formed thereon. The second coating material 140 hasan index of refraction that is less than the index of refraction of thecladding material 120, such that any light energy that escapes the core110 into the cladding material 120 is confined within the claddingmaterial 120. Accordingly, the energy confined within the claddingmaterial 120 may be guided to a downstream location where it is moredesirable to extricate the energy, as will be described below. Thesecond coating material 140 may comprise commercially availablematerials having an index of refraction less than the index ofrefraction of the cladding material 120. An exemplary coating isdisclosed in co-assigned U.S. Pat. No. 5,822,489 to Hale, which isincorporated herein by reference.

Turning to FIG. 2, illustrated is a plan view of another embodiment ofan optical transmission fiber 200 constructed according to theprinciples of the present invention. The optical transmission fiber 200includes a first fiber portion 200 a and a second fiber portion 200 b.The first fiber portion 200 a includes a first core portion 210 a, afirst cladding material portion 220 a around the first core portion 210a, and a first coating material 230 around the first cladding materialportion 220 a. As with the optical transmission fiber 100 shown in FIG.1, the first coating material 230 has an index of refraction that isgreater than an index of refraction of the first cladding materialportion 220 a. The second fiber portion 200 b includes a second coreportion 210 b, a second cladding material portion 220 b and a secondcoating material 240 around the second cladding material portion 220 b.The first and second core portions 210 a, 210 b, the first and secondcladding material portions 220 a, 220 b, and the first and secondcoating materials 230, 240 may have the same materials and constructionas the related features shown in FIG. 1. Accordingly, the second coatingmaterial 240 has an index of refraction that is less than an index ofrefraction of the second cladding material portion 220 b.

The optical transmission fiber 200 also includes an optical device 250.The optical device 250 may be any conventional device that can be usedto join two fibers end-to-end. For example, the optical device 250 maybe a fusion splice, optical connector or other optical coupler thatjoins two fiber terminations without altering a transmission signalpropagating therethrough. Those skilled in the art understand that thereare myriad other optically passive or transparent means for joining twofibers end-to-end within the scope of the present invention.

Those skilled in the art also understand that junctions between opticaltransmission fibers and/or optical components typically include one ormore types of surrounding material, fusion splice protectors, protectorsleeves or connector ferrules exterior to the junction to providemechanical robustness and environmental protection. Such surroundingmaterial 270 is generally indicated in FIG. 2.

As a transmission signal passes from the first core portion 210 a to thesecond core portion 210 b through the optical device 250, anymisalignment between the core portions 210 a, 210 b can cause couplinglosses. Of course, other factors may also influence the couplingefficiency of the junction between the core portions 210 a, 210 b andthe optical device 250. The light energy that fails to couple to thesecond core portion 210 b may be injected into the cladding materialportions 220 a, 220 b.

The light energy entering the first cladding material portion 220 a willquickly diffuse out of the first fiber portion 200 a because the firstcoating material 230 has a higher index of refraction that the firstcladding material portion 220 a. It has presently been recognized thatas the stray light energy passes through the first coating material 230and the surrounding material 270, the light energy will at leastpartially convert to heat energy. It has also been presently recognizedthat in high-power applications the excessive heat energy resulting fromthe poor coupling of the light energy to the core (210 a, 210 b) damagesthe transmission fiber, which can result in catastrophic failure of thefiber long before the fiber's design life. Moreover, because couplinglosses and other factors leading to the injection of light into thecladding portion of a transmission fiber frequently occur proximate orwithin optical components (e.g., bulk optics packages and modulators),the excessive heat can damage or destroy the optical components inaddition to the transmission fiber.

Returning to FIG. 2, coupling losses and other factors may also causestray light energy to be injected into the second cladding materialportion 220 b. However, because the second coating material 240 has anindex of refraction that is less than that of the second claddingmaterial portion 220 b, the stray light energy is confined within thesecond cladding material portion 220 b. Accordingly, the unwanted energymay be guided downstream (in the direction of arrow 260) to a locationwhere extraction is more desirable. Exemplary means for such extractionis more fully discussed below.

Turning to FIG. 3, illustrated is a plan sectional view of anotherembodiment of the optical transmission fiber 200 shown in FIG. 2.However, in the embodiment shown in FIG. 3, an active optical device 310couples the first and second fiber portions 200 a, 200 b instead of theoptically transparent optical device 250 shown in FIG. 2. The activeoptical device 310 intentionally alters the transmission signalpropagating in the general direction of the arrow 260. For example, theoptical device 310 may be an amplifier, a modulator, a bulk opticspackage or its components, a multiplexer or a tapered-fiber bundle. Inthe particular embodiment shown, the optical device 310 is aconventional bulk optics package that may comprise coupling optics 320(e.g., lenses and/or collimators) and an active optical component 330(e.g., an isolator or filter). The first fiber portion 200 a isoptically coupled to an input 340 of the optical device 310 and thesecond fiber portion 200 b is optically coupled to an output 350 of theoptical device 310.

Those skilled in the art understand that optically transparent couplingsare often difficult to achieve, even with embodiments similar to theembodiment shown in FIG. 2. For example, coupling losses can arise fromany misalignment of the joined fiber ends or optical components. As aresult, stray light energy may be undesirably injected into the claddingportion of a transmission fiber. With conventional transmission fibers,the exterior coating material has an index of refraction that is greaterthan the index of refraction of the cladding, such that the stray energyin the cladding passes to and through the exterior coating material. Thecoating material and any material exterior thereto (such as surroundingmaterial 360) then absorbs the light energy and converts it into heat.As discussed above, the excessive heat can cause severe damage to thetransmission fiber and surrounding optical components (such as opticaldevice 310) and cause the fiber and/or components to fail long beforetheir design lives, especially in today's high-power applications.

Turning to FIG. 4, illustrated is a plan view of another embodiment ofan optical transmission fiber 400 constructed according to theprinciples of the present invention. The optical transmission fiber 400includes a first fiber portion 400 a, a mode stripper 410, a heat sink420 and a second fiber portion 400 b. The first fiber portion 400 a maybe similar to the second fiber portions 200 b shown in FIGS. 2 and 3.Accordingly, the first fiber portion 400 a includes a cladding portion430 and a coating material 440, the coating material 440 having an indexof refraction that is less than an index of refraction of the claddingportion 430.

The mode stripper 410, which may be a conventional device, decoupleslight energy propagating in the cladding portion 430 and directs thelight energy away from the fiber 400. For example, the mode stripper 410may direct the unwanted light energy into the heat sink 420, where thelight energy may be converted into heat energy and radiated into theambient environment. The heat sink 420 may comprise a thermallyconductive metal or other materials capable of withstanding hightemperatures and configured to radiate or otherwise disperse heat energyinto the ambient environment. In one embodiment, the heat sink 420 maymerely comprise the air in the ambient environment.

The mode stripper 410 and heat sink 420 are located distal from anyoptical components (such as the optical device 250 shown in FIG. 2 orthe optical device 310 shown in FIG. 3), such that the dispersed heatdoes not damage the optical components. Moreover, because the heatenergy is extracted by the mode stripper 410 and/or heat sink 420, whichare designed to accommodate high temperatures, the transmission fiber400 is not damaged by the heat energy. Thus, by effective thermalmanagement within the transmission fiber 400, the integrity of the fiber400 is maintained despite the partial injection of the high-poweredtransmission signal into the cladding portion 430.

As shown in the embodiment illustrated in FIG. 4, the opticaltransmission fiber 400 may also include a fiber portion 400 b thatincludes a curved portion or bend 450 having a radius or kink smallerthan about 10 mm. Those skilled in the art understand that whileconventional installation procedures generally advise against suchbends, these bends are sometimes unavoidable, such as in the confinedspace of conventional switching panels. One reason the tight bends inconventional transmission fibers are avoided is that the angle ofincidence of the transmission signal propagating therethrough can becomeless than the critical angle of incidence, such that a portion of thetransmission signal may be injected into the cladding material. Asdiscussed above, the high-powered light energy propagating within thecladding material can result in excessive temperatures in and around thefiber. The excessive temperatures are particularly undesirableconsidering that bends such as the bend 450 shown in FIG. 4 aretypically located proximate optical components that are not designed towithstand excessive temperatures.

To prevent excessive heating of the fiber 400, the embodiment shown inFIG. 4 also includes a second heat sink 460 downstream of the bend 450.The second fiber portion 400 b also includes cladding material 470 and acoating material 480, wherein the coating material 480 has an index ofrefraction that is less than an index of refraction of the claddingmaterial 470. Due to the lower index refraction of the coating material480, the light energy injected into the cladding material 470 at thebend 450 is guided downstream to the heat sink 460. The heat sink 460may comprise a thermally conductive compound, such as RTV silicon.Moreover, a portion 490 of the coating under the heat sink 460 may havean index of refraction that is greater than the index of refraction ofthe cladding material 470 thereunder, thereby allowing energy to passfrom the cladding 470 to the heat sink 460. The light energy escapes thesecond fiber portion 400 b into the heat sink 460, where it is convertedinto heat energy and dissipated into the ambient environment. Of course,those skilled in the art understand that the heat sink 420 in the firstfiber portion 400 a and the heat sink 460 in the second fiber portion400 b are in general, interchangeable devices, and that other heatdissipation means are also within the scope of the present invention.

Thus, the present invention provides means for guiding light energyundesirably injected into the cladding portion of an opticaltransmission fiber to a location where it is more desirable to removethe light energy. Accordingly, critical portions of the transmissionfiber and/or optical components may be located distal from any damagingtemperatures. More specifically, the present invention provides aportion of an optical transmission fiber with a coating that has a lowerindex of refraction that the underlying cladding material. The coatingconfines the stray light energy within the cladding material until thelight energy can be safely removed without damaging the opticaltransmission fiber or optical components coupled thereto. The presentinvention also provides means for effectively removing the stray lightenergy by at least partially converting it into heat energy anddissipating the heat energy by a mode stripper and/or heat sink, whichmay comprise air or other thermally conductive materials. Accordingly,optical transmission fibers constructed according to the principles ofthe present invention are protected from the high temperatures arisingfrom the injection of high-powered transmission signals into thecladding material.

While optical fibers comprising a coating having a refractive index thatis less than the refractive index of the cladding do currently exist,such fibers are only used on cladding-pumped fiber amplifiers andlasers, such as those used in conjunction with tapered fiber bundles.The coating having a lower refractive index than that of the claddingmaterial is used to confine signals from pump diodes intentionallyinjected directly into the cladding. However, such coatings do notpresently exist in transmission optic fiber. In fact, it iscounter-intuitive to those skilled in the art to employ such a lowerrefractive index coating on optical transmission fibers, such as, thoseof the present invention, because it is desirable to allow any lightenergy escaping the core of an optical transmission fiber to exit thecladding as quickly as possible to avoid any interference between this“lost” energy and the remaining signal energy propagating in the core orcladding.

Turning briefly to FIG. 5, illustrated is a plan view of an embodimentof an optical transmission system 500 constructed according to theprinciples of the present invention. The optical transmission system 500includes an optical transmission fiber 510, which may be similar to theoptical transmission fiber 100 shown in FIG. 1, the optical transmissionfiber 200 shown in FIG. 2 or 3, the optical transmission fiber 400 shownin FIG. 4, or a combination thereof. The optical transmission system 500also includes a transmitter 520 and a receiver 530 coupled together bythe optical transmission fiber 510. The transmitter 520 and receiver 530may be directly coupled by the optical transmission fiber 510. In otherembodiments, the transmitter 520 and receiver. 530 may be indirectlycoupled by the optical transmission fiber 510, such that additionoptical fibers or components may also be coupled between the transmitter520 and receiver 530.

Although certain embodiments of the present invention have beendescribed in detail, those skilled in the art should understand thatthey can make various changes, substitutions and alterations to thoseembodiments without departing from the spirit and scope of theinvention.

1-14. (canceled)
 15. A method, comprising: removing energy from anoptical transmission fiber comprising: locating a cladding materialaround a core, said core having a first index of refraction, saidcladding material having a second index of refraction less than saidfirst index of refraction; locating a coating material around saidcladding material, said coating material having a third index ofrefraction less than said second index of refraction; and removing saidenergy from said optical transmission fiber with an energy removaldevice located adjacent said coating material.
 16. The method as recitedin claim 15 wherein said energy removal device comprises a modestripper.
 17. The method as recited in claim 15 wherein said energyremoval device comprises a heat sink coupled to said coating material.18. The method as recited in claim 17 wherein said heat sink comprises athermally conductive compound formed around said coating material. 19.The method as recited in claim 17 wherein said energy removal device isan ambient environment around said coating material.
 20. The method asrecited in claim 15 wherein said energy is selected from the groupconsisting of: light energy; and heat energy.
 21. The method as recitedin claim 15, wherein said coating comprises a first coating materialaround a first portion of said cladding and a second coating materialaround a second portion of said cladding, wherein said second coatingmaterial has said third index of refraction, and said first coatingmaterial has a fourth index of refraction greater than said second indexof refraction.
 22. The method as recited in claim 21, wherein saidsecond coating material substantially confines said energy escaping fromsaid core within said first portion of said cladding material fortransmission to said energy removal location
 23. The method as recitedin claim 21, wherein said energy removal device is located adjacent saidfirst coating material.
 24. The method as recited in claim 15, whereinsaid optical transmission fiber couples a transmitter to a receiver inan optical transmission system.
 25. The method as recited in claim 24,wherein said optical transmission system includes said energy removaldevice.
 26. The method as recited in claim 15, wherein said opticaltransmission fiber is optically coupled to an optical device.
 27. Themethod as recited in claim 25, wherein said optical device transmits atransmission signal carried by said optical transmission fiber withoutaltering said transmission signal.